Thermoplastic sheets or webs of material are often formed by extruding heated thermoplastic ("melt") through a die and depositing same as a film on a cooling or casting roll so as to form a sheet of a desired thickness.
A problem associated with the extrusion process is web stability. The film normally exits the die many times thicker than its finished form and must be stretched while in the molten state. If a film is stretched past a certain limit it will no longer stretch uniformly, resulting generally in a thick/thin pattern near the ends of the web. Similarly, a temperature gradient within the melt exiting the die can result in a cyclic thick/thin pattern and unstable edges as the hotter melt portion of the exiting web will stretch more than the cooler portion.
The stretching of the melt thins the melt curtain in one direction but also contributes to "neck-in" wherein the web narrows in the transverse direction perpendicular to the stretching direction, this condition can create an edge bead which interferes with uniform roll contact and uniform rate of cooling. Also, this condition may be accompanied by edge instability wherein strains are introduced that may contribute to quality problems in downstream process steps.
In addition, the greater the melt necks in, the greater the amount of waste edge material which must be trimmed. This results in less material to sell.
Finally, during production, the rapidly moving cooling roll carries entrained air on the surface thereof which is trapped between the melt and roll and which prevents intimate contact between the two, allows continued stretching of the web as it passes around the roll and adversely affects surface finish.
It has been found that if the melt is placed in intimate contact with the roll surface neck-in and other adverse effects are reduced. One method of attempting to provide intimate contact has involved the use of an air knife which impinges a high velocity curtain of air against the web thereby pinning same against the roll surface. Often used simultaneously with the air knife are air edge pinners which use a narrow focused jet of air to pin and stabilize the edge bead just prior to the air knife. Forced air pinning is, however, of limited use during high speed operation due to film distortions created by air being trapped under the molten web as the web is laid onto the cooling roll.
In addition, electrostatic pinners have been employed to attempt to force the web into intimate roll contact. These pinners have also been limited to slower operation, as air bubbles can be trapped under the web at high speeds.
Still further, vacuum boxes have been used to attempt to provide intimate web/roll contact. This technology employs an evacuated chamber placed immediately upstream of the die lips. The air carried by the roll surface is removed and a slight negative pressure is created on one side of the melt. Atmospheric pressure acting on the other side of the melt then forces the molten plastic into contact with the roll surface. Though this technique has been useful, it has not been as efficient or effective as required.